For many years, optical modulators have been made out of electro-optic material, such as lithium niobate. Optical waveguides are formed within the electro-optic material, with metal contact regions disposed on the surface of each waveguide arm. A continuous wave (CW) optical signal is launched into the waveguide, and an electrical data signal input is applied as an input to the metal contact regions. The applied electrical signal modifies the refractive index of the waveguide region underneath the contact, thus changing the speed of propagation along the waveguide. By applying the voltage(s) that produce a 7C phase shift between the two arms, a nonlinear (digital) Mach-Zehnder modulator is formed.
Although this type of external modulator has proven extremely useful, there is an increasing desire to form various optical components, subsystems and systems out of semiconductor material systems (e.g., InP, GaAs, silicon, or the like), with silicon-based platforms being generally preferred. It is further desirable to integrate the various electronic components associated with such systems (for example, the input electrical data drive circuit for an electro-optic modulator) with the optical components on the same silicon substrate. Clearly, the use of lithium niobate-based optical devices in such a situation is not an option. Moreover, it is well-known that lithium niobate-based devices have inherent performance limitations at data rates exceeding, for example, 1 GB/s, since they need to be modeled as traveling wave structures, with relatively complex electrical drive structures required to attempt to have the device operate at the requisite speed.
A significant advance has been made in the ability to provide optical modulation in a silicon-based platform, as disclosed in U.S. Pat. No. 6,845,198 issued to R. K. Montgomery et al. on Jan. 18, 2005, assigned to the assignee of this application and incorporated herein by reference. FIG. 1 illustrates one exemplary arrangement of a silicon-based modulator device as disclosed in the Montgomery et al. patent. In this case, a silicon-based optical modulator 1 comprises a doped silicon layer 2 (typically, polysilicon) disposed in an overlapped arrangement with an oppositely-doped portion of a sub-micron thick silicon surface layer 3 (often referred to in the art as an SOI layer). SOI layer 3 is shown as the surface layer of a conventional silicon-on-insulator (SOI) structure 4, which further includes a silicon substrate 5 and a buried oxide layer 6. Importantly, a relatively thin dielectric layer 7 (such as, for example, silicon dioxide, silicon nitride, potassium oxide, bismuth oxide, hafnium oxide, or other high-dielectric-constant electrical insulating material) is disposed along the overlapped region between SOI layer 3 and doped polysilicon layer 2. The overlapped area defined by polysilicon layer 2, dielectric 7 and SOI layer 3 defines the “active region” of optical modulator 1. In one embodiment, polysilicon layer 2 may be p-doped and SOI layer 3 may be n-doped; the complementary doping arrangement (i.e., n-doped polysilicon layer 2 and p-doped SOI layer 3) may also be utilized.
FIG. 2 is an enlarged view of the active region of modulator 1, illustrating the optical intensity associated with a signal propagating through the structure (in a direction perpendicular to the paper) and also illustrating the width W of the overlap between polysilicon layer 2 and SOI layer 3. In operation, free carriers will accumulate and deplete on either side of dielectric layer 7 as a function of the voltages (i.e., the electrical data input signals) applied to doped polysilicon layer 2 (VREF2) and SOI layer 3 (VREF3). The modulation of the free carrier concentration results in changing the effective refractive index in the active region, thus introducing phase modulation of an optical signal propagating along a waveguide defined by the active region. In the diagram of FIG. 2, the optical signal will propagate along the y-axis, in the direction perpendicular to the paper.
FIG. 3 illustrates an exemplary prior art silicon-based Mach-Zehnder interferometer (MZI) 10 that is configured to utilize silicon-based modulating devices 1 as described above. As shown, prior art MZI 10 comprises an input waveguide section 12 and an output waveguide section 14. A pair of waveguiding modulator arms 16 and 18 are shown, where in this example waveguide arm 16 is formed to include a modulating device 1 as described above.
In operation, an incoming continuous wave (CW) light signal from a laser source (not shown) is coupled into input waveguide section 12. The CW signal is thereafter split to propagate along waveguide arms 16 and 18. The application of an electrical drive signal to modulator 1 along arm 16 will provide the desired phase shift to modulate the optical signal, forming a modulated optical output signal along output waveguide 14. A pair of electrodes 20 are illustrated in association with modulator 1 and used to provide the electrical drive signals (VREF2, VREF3). A similar modulating device may be disposed along waveguiding arm 18 to likewise introduce a phase delay onto the propagating optical signal. When operating in the digital domain, the electrodes may be turned “on” when desiring to transmit a logical “1” and then turned “off” to transmit a logical “0”.
FIG. 4 is a diagrammatic illustration of modulator 10, illustrating the various electric field components associated with the prior art modulator, defining the chirp parameter which is the specific subject matter of concern in the present invention. Referring to FIG. 4, the incoming CW optical signal is defined by the electrical field Ein. Presuming a 50:50 power split into waveguide arms 16, 18, each waveguide will see an electric field of Ein/√{square root over (2)} (also shown as EL and ER) at their respective inputs. Each propagating signal will modulated along its respective arm, in the manner described above, and the electric fields of the output signals exiting waveguide arms 16, 18 are expressed as follows:Eleft=eiθLEL, andEright=eiθRER.
Combining these two signals along output waveguide 14 yields the following value for the output electrical field Eout:
                              E          out                =                ⁢                              1                          2                                ⁢                      (                                          E                left                            +                              E                                  right                  ⁢                                                                                                              )                                                            =                    ⁢                                    E                              i                ⁢                                                                  ⁢                n                                      ⁢                          cos              ⁡                              (                                  Δ                  ⁢                                                                          ⁢                  ϕ                                )                                      ⁢                          ⅇ                              ⅈ                ⁢                                                                  ⁢                ϕ                                                    ,            where Δφ=(θR−θL)/2 and φ=(θR+θL)/2. The cos(Δφ) term is associated with the amplitude modulation imparted onto the propagating optical signal by virtue of the applied electrical input signal The eiφ term is a “pure” phase term, representative of the overall phase remaining in the output signal when compared to the input signal.
To the first order, the output power Pout of a conventional modulator as shown above is given by the equation:Pout=|Eout|2=½|Ein|2[1+cos(θR−θL)]where the optical output power level is controlled by changing the value of the net phase difference Δφ between the two arms. FIG. 5 is a plot of this relationship, illustrating the output power as a function of phase shift between the two arms (a “1” output associated with maximum output power Pout and a “0” output associated with minimum output power Pout). That is, a differential phase shift between the two arms of the modulator provides either constructive interference (e.g., “1”) or destructive interference (e.g., “0”). As will be described below, a modulator may also include a DC section to optically balance the arms and set the operating point at a desired location along the transfer curve shown in FIG. 5.
While considered a significant advance in the state of the art over lithium niobate modulators, silicon-based optical modulators in general and the exemplary configuration of FIG. 3 in particular are known to suffer from chirp as a result of the inherent phase response and optical loss differences between the two arms of the modulator. Chirp is a time-varying optical phase that can be detrimental to the transmission behavior of an optical signal as it propagates through dispersive fiber. The chirp behavior of optical modulators is often characterized using an “alpha parameter” that is defined as the amount of phase modulation normalized to the amount of amplitude (intensity) modulation produced by the modulator. The alpha (α) parameter may be defined as follows:
                    α        =                ⁢                  2          ⁢                                          ⁢                                                    ⅆ                ϕ                                            ⅆ                t                                                                    1                P                            ⁢                                                ⅆ                                      P                    ′                                                                    ⅆ                  t                                                                                                      =                    ⁢                                    -                              1                                  tan                  ⁡                                      (                                          Δ                      ⁢                                                                                          ⁢                      ϕ                                        )                                                                        ⁢                          (                                                                                          ⅆ                                              θ                        R                                                                                    ⅆ                      t                                                        +                                                            ⅆ                                              θ                        L                                                                                    ⅆ                      t                                                                                                                                  ⅆ                                              θ                        R                                                                                    ⅆ                      t                                                        -                                                            ⅆ                                              θ                        L                                                                                    ⅆ                      t                                                                                  )                                      ,            and may exhibit a value that is zero, positive or negative, where for “zero” chirp, it is required that dθR/dt=−dθL/dt. In some applications, however, it is desirable to have a small amount of negative chirp (i.e., a small negative alpha parameter) to extend the transmission distance of a signal along a dispersive medium, such as an optical fiber, before dispersion limits the range. Even if “desirable”, there is still a need to control (or “know”) the amount of chirp that is associated with a particular modulator.
Conventional silicon-based optical modulators are known to exhibit non-zero chirp (even when configured in a symmetric drive arrangement) as a result of the nonlinear phase versus “applied voltage” response of their structure, as shown in FIG. 6. Increasing either the modulation speed or the distance traveled by the modulated optical signal has been found to only exacerbate the chirp problem, since the dispersion characteristics of the transmission fiber will have an even greater impact.
Thus, a remaining need in the design of silicon-based optical modulators is a way of controlling the chirp that is created during the modulation process and, indeed, creating a “desired” value of chirp for a specific application/system configuration.